The classic renderer uses ray tracing for all the primary rendering of geometry, but the lighting is still processed through texture-based light maps like the original game, meaning you get similar lighting and ambience, with some intriguing RT effects: water surfaces are completely different, with RT reflections that show the full surroundings. You also get RT-specific view distortion effects when under water too. A really nice upgrade comes from the teleporters, which now show you the area you'll arrive in when you travel through, Portal-style.

The path-traced option offers a more radical transformation by leveraging path tracing for all of the game's lighting, full replacing light maps and the other tricks Quake used to simulate the look of light back in 1996. Indoor scenes are lit by torches and other small light sources, casting dynamic shadows from appropriate objects. As a part of this system, all muzzle flashes from gunfire cast light and shadows as well, leading to some pretty awesome moments in the dark, reminiscent of Doom 3. One of my favourite RT elements is the lightning gun, which casts light all along the surface of the lighting trail as it leaves the weapon's muzzle.